The present invention relates to a dynamic pressure bearing, and particularly to a dynamic pressure bearing having dynamic pressure grooves formed on the inner peripheral surface of a sleeve in which a shaft is fitted, which has high precision, reduces the manufacturing cost and has a lower bearing damage rate, and a method of manufacturing the dynamic pressure bearing.
A dynamic pressure bearing, in which a shaft is fitted in a sleeve and relatively rotated and herringbone type or spiral type grooves are formed on at any one of the inner peripheral surface of the sleeve and the outer peripheral surface of the shaft, is used as a bearing apparatus for a spindle motor rotating at high speed in information equipment or the like.
For example, as shown in FIG. 9, there is used such a dynamic pressure bearing that herring bone type dynamic pressure grooves 23 are formed on the inner peripheral surface of a sleeve 21 on which a stator coil 21 is mounted, a shaft 25 is fitted in the sleeve 21 so that a ferromagnetic material (not shown) is disposed to face the stator coil 20 and the stator coil 20 is supplied with current to rotate the shaft 25 at high speed. Such a dynamic pressure bearing is used in an apparatus in which a polygon mirror or optical disc (not shown) is fixed to the shaft 25 and a laser beam is reflected therefrom to record or read out information.
On the inner peripheral surface 21b of the sleeve 21, a bearing portion 22 is equipped at a step portion 21a having a slightly reduced diameter, and herringbone type or spiral type V-shaped dynamic pressure grooves 23 are formed on the inner surface of the bearing portion 22 of the step portion 21a as shown in FIGS. 10 and 11. Alternatively, dynamic pressure grooves may be formed on the outer peripheral surface of the shaft 25 in place of formation of the dynamic pressure grooves 23 on the inner peripheral surface of the sleeve 21. Such dynamic pressure grooves 23 are usually formed by plastic working based on form rolling. Further, there is known an example of forming dynamic pressure grooves by using an electrolytic etching working in order to satisfy recent requirements for high precision and increase of working speed. In the case of stainless steel, etc., etching working on these materials needs much time. Therefore, there has been proposed a dynamic pressure bearing achieved by subjecting a bearing face of a sleeve of aluminum alloy to plastic working based on ball form rolling and carrying out an alumite treatment after honing working, thereby forming an anode oxidation coating (Japanese Laid-open Patent Application No. Hei-8-247138).
To overcome lack of lubricating oil in bearing gaps to enhance durability and bearing rigidity, such a dynamic pressure bearing is proposed that it includes dynamic pressure grooves formed on the inner peripheral surface of the sleeve, lubricating oil or grease is impregnated in the bearing body 31, and dynamic pressure grooves 33 are formed to be inclined with respect to the bearing face of the bearing body as shown in FIG. 12A (Japanese Laid-open Patent Application No. Hei-11-82479). According to the bearing apparatus thus constructed, the rotational shaft is supported under anon-contact state by dynamic pressure films of the lubricating oil formed in the bearing gaps, and the lubricating oil is circulated between the inside of the bearing body 32 and each bearing gap through opening portions of the bearing face. In this case, as shown in FIG. 12B, each dynamic pressure groove 33 comprises one oblique groove 33a, the other oblique groove 33b and a smoothing portion 34 formed between the oblique grooves 33a and 33b. 
As described above, the dynamic pressure grooves 23 of the bearing portion 22 equipped at the step portion 21a of the inner peripheral surface of the conventional sleeve 21 are formed over the whole width D from one end portion 21c of the step portion 21c to the other end portion of the step portion 21c. Therefore, as shown in FIG. 11, particularly when the shaft 25 is rotated in a bell-shaped mode (conical mode or precession mode) at the start or stop time of the motor, the shaft 25 damages the end portions 21c of the step portion 21a. If the end portions 21c of the step portion 21a at which the bearing portion 22 is equipped are repetitively damaged due to rotation of the shaft 25 in such a conical mode as described above, the performance as the dynamic pressure bearing is degraded, and finally it adversely affects the performance and lifetime of equipment equipped with such a dynamic pressure bearing.
According to the conventional technique, after dynamic pressure grooves 23 are formed on the inner peripheral surface of a sleeve by the plastic working based on the form rolling as described above, it is required to carry out finish machining by reaming, balling, horning or the like, so that the manufacturing cost is high.
The etching process used as a work to form dynamic pressure grooves has high precision, however, it still needs a high manufacturing cost. Further, dynamic pressure grooves may be formed by the electrolytic etching processing. However, the conventional processing has a problem that pattern symmetry is lost unless the positioning to a work is not performed at high precision, so that fluid flow occurs.